Warren Hill, 52, is scheduled to die by a single injection of the sedative pentobarbital at 7pm tonight. The state of Georgia is pressing ahead with preparations for the execution despite a nationwide outcry that Hill is "mentally retarded" and therefore at risk of wrongful execution. He was put on death row for killing a fellow prisoner, Joseph Handspike, in 1990, at a time when he was already serving a life sentence for murdering his girlfriend Myra Wright.

Here are 10 reasons why the judicial killing of Hill would be inappropriate:

1. The US supreme court in 2002 banned executions for prisoners who are "mentally retarded" – in other words, those with learning difficulties. The justices ruled that such executions violated the eighth amendment of the US constitution that prohibits cruel and unusual punishments.

2. Just months after the supreme court ban was imposed, a Georgia court found that Hill was "mentally retarded" by a "preponderance of the evidence". In plain English, he was likely to be "mentally retarded" and fall into the very category of prisoner who the supreme court had just declared must not be executed.

3. Paradoxically, in 1988 Georgia became the first state in America to ban the execution of mentally disabled prisoners. It made the move in response to huge public outcry following its judicial killing of an intellectually disabled prisoner, Jerome Bowden. Days before he was put to death Bowden was tested and found to have the mental capacity of a 12-year-old. History now appears to be repeating itself.

4. When Georgia banned executions of the "mentally retarded", it applied a standard to such cases that meant that death row prisoners had to prove "beyond a reasonable doubt" that they had learning difficulties. Experts in intellectual impairment say that is virtually impossible to do. No other state in the US applies such a harsh standard. Most states use the "preponderance of evidence" proof that Hill was found to meet.

5. As Leah Sears, the former chief justice of Georgia's supreme court – the highest member of the highest judicial panel in the state – herself pointed out the "beyond a reasonable doubt" standard means that the "state may still execute people who are in all probability mentally retarded. The state may execute people who are more than likely mentally retarded. The state may even execute people who are almost certainly mentally retarded."

6. Hill shows many of the classic signs of intellectual disability. He has a family history of learning difficulties and borderline intellectual functioning, though the jury at his 1991 trial heard little about that. In terms of IQ, he tests 70 – a sub-average level of functioning shared by only 3% of the general population. Such challenges render him subject to impulsive behaviour and poor decision making at times of stress and threat.

7. The family of his victim, Joseph Handspike, has never been consulted by the Georgia prosecutors about the application of the death penalty in this case.

8. The Handspike family is opposed to Hill being executed. Handspike's nephew Richard Handspike has written on behalf of the family that : "I and my family feel strongly that persons with any kind of significant mental disabilities should not be put to death."

9. Several jurors at Hill's original trial have said they would have sentenced him to life imprisonment without parole – the sentence that his lawyers are now pleading for – had the option been open to them.

10. To execute Hill could be a violation of international law. Christof Heyns, UN special rapporteur on extrajudicial, summary or arbitrary executions has said that it would be a "fatality in violation of international as well as domestic law" and a "violation of death penalty safeguards".